1. Field of Invention
The present invention relates in general to microlenses of the type which can be used in a micro-optical system such as optical communication or the like. More particularly, the invention provides a gradient refractive index type anamorphic planar microlens. The microlens has a semiellipsoidal refractive index distribution region which is formed in a transparent substrate so as to have a major axis and a minor axis on a surface of the transparent substrate. The present invention is directed to both the microlens structure and a method of producing a gradient refractive index type anamorphic planar microlens.
2. Description of the Prior Art
As shown in FIG. 1, a gradient refractive index type planar microlens has a plate-like semitransparent substrate 1 in which are formed several refractive index distribution regions 2 each having a "lens" effect (acting as a lens). Each region 2 converges light as shown by arrows 3. The opposite surfaces of the lens are flat. This permits the lens to be more easily used than common spherical surface lenses or a non-spherical surface lens. A plurality of regions each acting as a "lens" can be simultaneously formed on the same substrate.
There has been proposed a lens in which the refractive index distribution regions are shaped to be semispherical, or semicylindrical.
Semiconductor lasers are widely used as an optical communication system light source. However, there occurs astigmatism in a beam 6 radiated from a semiconductor laser 4 as shown in FIGS. 2A and 2B. The beam is shown as being elliptical in section because the radiation angle is different in its expanding angles between the directions perpendicular to and parallel to an active layer 5. For use in an optical lens system to collimate such a beam as described above, a flat plate microlens having a semispherical refractive index distribution region is not optimum.
To collimate light rays having such elliptical expansion as described above into circular parallel light rays, it is possible to think about an optical system in which semicircular lenses are combined to be perpendicular to each other, however, the optical system becomes complicated with respect to the curvature and thickness of the lenses used in the optical system to collimate the light and therefore it is quite difficult to produce the lenses. For example, in the case of using a flat plate microlens having a semicylindrical refractive index distribution regions, it is necessary that lenses which are different in numerical aperture (N.A.) be placed one on the other so as to be perpendicular to each other.